Distributed power sources are used to feed additional active power into a utility grid close to electrical loads or to ensure standby power for critical loads when power from the grid is temporarily unavailable. Distributed power sources are connected to the utility grid or loads by power electronic switches.
When one or more distributed power sources become isolated from the rest of the power system and inadvertently continue to serve local island loads separately from the utility grid, the condition is known as a “loss of mains” or “unintentional islanding”.
Upon having lost the stability provided by the utility grid, differences in active and reactive power consumed by the loads in the local island and the power generated by the distributed power source can lead to sudden drastic voltage and/or frequency changes in the local grid. These drastic voltage and/or frequency changes usually cause a distributed power source protection device to act and disconnect immediately.
If, however, the active and the reactive power of the loads in the local island matches the active and the reactive power generated by the distributed power source, no change in voltage and/or frequency can occur at all that would trigger the passive protection device of the distributed power source. Unless an active islanding detection is provided, the distributed power generator continues to operate. Even though some of the distributed power generators are designed to operate in an islanding grid, a number of potentially serious problems is associated with islanding.
Distributed power sources can become an isolated source of electricity power during loss of mains, causing harm to utility personnel and equipment.
Customer equipment can be damaged by uncontrolled voltage and/or frequency changes in the local grid.
Utility personnel or the public can be harmed by the inadvertent energizing of lines by the distributed power sources.
It is therefore desirable to be able to immediately detect an unintentional loss of mains in order to be able to disconnect the distributed power source from the network.
EP patent publication 1764894 discloses a method for detecting islanding operation of a distributed generator. The method includes the steps of introducing a reactive current reference square wave to the grid, detecting load voltage changes at every change in the reactive current reference wave, and determining whether the detected load voltage changes exceed a predetermined islanding detection threshold value, indicating a loss of mains and an islanding operation of the distributed generator. The amplitude of the reactive current reference square wave is constant.